Micromechanical components based on silicon, for example, sensors or micromirrors, are generally made up of one or multiple conductive functional layers of silicon. The areas of the functional layers that represent the movable part of the MEMS are situated directly on a sacrificial layer. The non-movable areas are, in contrast, connected directly on the substrate without a sacrificial layer. Movable and fixed areas are connected with one another via a suitable suspension. At the end of the manufacturing process, the sacrificial layer is selectively removed with the aid of a suitable isotropic etching method, which makes the component functional. Wet chemical methods are often not usable here, since very closely adjacent functional elements become stuck during the subsequent drying of the MEMS structure. Plasma-assisted or plasma-free isotropic etching methods are necessary, in which exclusively gaseous reactants and products are participants.
Very different types of silicon oxide are common materials for sacrificial layers. Due to the extreme layer stress of silicon oxide in combination with silicon as a functional material, only very thin sacrificial layers may be implemented (usually in the range of 0.1 μm to 2 μm). The selective removal of the sacrificial layer may take place here, for example, with the aid of HF gas phase etching. In this case, however, the etching rate is limited, causing the maximum usable volume of the sacrificial layer to be limited.
One variant is the use of epitaxial polysilicon as the sacrificial material. High deposition rates favor the formation of high-volume sacrificial structures. Isotropic etching methods are known which may be used to remove the sacrificial structures using very high etching rates. Both plasma-assisted and plasma-free etching methods are suitable for this. In both cases, polysilicon may be etched at high selectivity compared to usual mask materials such as silicon oxide, silicon nitride, aluminum or photoresist.
A particular focus is on the design of 3D MEMS structures, in which the functional structure and the sacrificial structure are made up of the same material (e.g., epitaxial polysilicon). The functional structure is protected against etching attacks on all surfaces by suitable passivation materials (e.g., SiO2). If this technology is applied, buried sacrificial structures of arbitrary complexity extending broadly laterally and vertically may be created. A great challenge is to completely remove the sacrificial structures in the isotropic etching step.
Plasma-assisted and plasma-free etching methods are fundamentally different. Purely chemical plasma-free etching is very well suited for the removal of buried sacrificial material. Compounds such as XeF2, CIF, CIF3, CIF5, BrF3, BrF5, IF5, IF5 etch silicon spontaneously, while common mask materials such as SiO2, Si3N4, SiON, silicon-rich nitrides or metals such as Al are etched very slowly (selectivity up to 1000). Great undercutting widths may be implemented here, even in extremely narrow sacrificial structures (<1 μm). Even at a great distance from the access opening in the mask, the etching rate remains nearly constant. There is also nearly no dependence of the etching rate on the size of the access. However, the etching rate is often limited by the vapor pressure of the used chemical (only approximately 3 torr in the case of XeF2). If too much sacrificial material is offered openly, the etching rate drops sharply. In other words, the volume etching rate is low (typically 11 mm3/min). If MEMS components having a high-volume sacrificial structure and a large open surface area >10% are to be etched, long etching times must be expected.
Plasma-assisted isotropic etching using fluorine compounds such as F2, SF6, CF4 or NF3 is also suitable for the removal of buried sacrificial material. In this case, the fluorine compound is activated in the plasma; the free fluorine radicals etch silicon spontaneously and without additional activation energy using ion bombardment.
Mask materials such as SiO2, Si3N4, SiON, silicon-rich nitrides or metals such as AL are only etched very slowly (selectivity >1000). Only by supplying activation energy by ion bombardment is it possible to remove these materials at higher etching rates. When silicon is etched, very high volume etching rates are possible (e.g., >500 mm3/min); however, the etching rate drops as the distance to the access opening increases. This is due to the fact that the radicals react not only with the silicon surface, but also with themselves, as a result of which the concentration and thus the etching rate decreases as the undercutting width increases. Furthermore, the etching rate depends strongly on the size of the access opening. The smaller the access, the lower the etching rate. Accordingly, the etching rate also drops sharply in the case of constrictions of the sacrificial structure. If MEMS components having a high-volume sacrificial structure and a large, open surface area >10% are to be etched, the major portion of the sacrificial silicon may be removed in a short etching time with the aid of a plasma-assisted method, as long as there is no limitation due to small etching accesses <20 μm.
If silicon is used as a sacrificial material in combination with arbitrary passivation material, the mask must be structured prior to the sacrificial layer etching. Silicon is released in this case. In air, a thin film of natural silicon oxide is always formed on the silicon surface (˜5 nm). To begin sacrificial layer etching, it is necessary to initially remove this natural oxide in a suitable manner. Plasma-assisted and plasma-free etching methods are again suitable for this purpose. If the passivation material is SiO2, the opening with the aid of CF4 plasma in combination with directed ion bombardment is common practice. In this case, the natural oxide on the surface may be selectively removed shortly before the sacrificial layer etching. A short isotropic HF gas phase etching step may also be used. Here, oxide is selectively and isotropically removed. If an interruption should occur during the sacrificial layer etching and the wafer is exposed to air, natural oxide will also form on buried sacrificial silicon on the etch fronts. This may result in an etching delay or even a complete stop of the etching progress when the sacrificial layer etching is resumed. In order to avoid this, it is no longer possible to work with plasma-assisted methods, since no ion bombardment is possible on buried structures. In order to provide a remedy, all that remains is an isotropic plasma-free etching step, e.g., HF gas phase etching for SiO2 as passivation.
This basically provides a sacrificial layer etching under a combination of plasma-assisted and plasma-free etching methods.
Generally, separate modules for plasma-free (described, for example, in German Patent Application DE 198 40 437 A1) or plasma-assisted etching are provided in the related art. The modules may then be linked with one another via a handling system. In this case, it would be possible to initially machine the wafer in the plasma module. In this case, the natural oxide on the surface could be opened and the isotropic plasma-assisted sacrificial layer etching could be started. After the etching was completed, the wafer would have to be unloaded and transferred into the next etching module. In this case, a further sacrificial layer etching could be carried out with the aid of a plasma-free etching method. Should natural oxide have been formed on buried sacrificial material for any reasons, the wafer would have to be unloaded again and transferred to a third module suitable for removing the natural oxide, e.g., HF gas phase etching. This results in high costs for at least three etching modules and a handling system in order to be able to hold the wafer in a vacuum between the individual process steps.
Moreover, long process durations result from multiple separate etching and handling steps and a distribution of the error possibilities, since all modules have to be available at the same time. In addition, there are increased maintenance costs because four system elements have to be serviced, a complication due to four different system elements including different software control, if necessary, and an increased risk of product damage due to multiple handling.
U.S. Pat. No. 6,221,784 B1 and PCT Application No. WO 02/095800 A2 describe etching modules in which plasma-assisted and plasma-free etching are to be combined. Here, structures are explicitly assumed in which useful material and sacrificial material differ. There is no way to use the plasma-activated species for isotropic etching at a high etching rate. Similarly, there is no way to remove buried natural oxide on etch fronts with the aid of gaseous HF.